Insufficient weather protection and ultraviolet degradation are problems, which are encountered when using solar reflectors made of a flexible specular silver mirror. When used outdoors, these mirrors must be durable and ultraviolet light (UV) resistant in order to retain their dimensional stability, aesthetic appearance, and specular-reflectance in the visible, ultraviolet, and near infra-red wavelengths.
Specular-reflectance is provided in a flexible silver mirror through a silvered composite lamina, having a thin layer of silver vapor-deposited on the surface of a flexible polyester substrate. Silver is the preferred metal because its reflectivity is substantially higher than that of other metals, such as aluminum. To retain specular reflectance over time, the prior art has focused on the application of advanced adhesives and protective films, layered over the polyester substrate and silver layer, to protect the mirrors from abrasion, weathering, and ultraviolet degradation.
Early techniques used to protect solar mirrors from abrasion, weathering, and ultraviolet degradation were developed with aluminum mirrors. For example, in U.S. Pat. No. 4,307,150, a solar reflector is disclosed wherein an opaque aluminum surface, vapor-deposited on a flexible polyester support-sheet, is protected from corrosion and weathering with an inter-polymer layer of acrylate or methacrylate copolymers. The support sheet consists of a biaxially oriented polyethylene terephthalate lamina having conventional slip agents, to facilitate winding, and a second polyethylene terephthalate lamina which contains no slip agent.
Silver is higher in specular reflection than aluminum. Thus, the logical assumption had been to substitute silver for aluminum in the solar reflector described above. However, this approach has been reported, in U.S. Pat. No. 4,645,714, to result in two undesirable phenomena. First, silver is susceptible to corrosion either through the development of pin holes in the acrylate coating or along peripheral portions of the silver-coated film. Second, a thin layer of silver, unlike a thin layer of aluminum, has a spectral window through which ultraviolet (“UV”) light readily passes. The peak transmission of this light is at 320 nm, and sunlight contains ultraviolet light at this wavelength. The transmission of ultraviolet light through the silver layer degrades the underlying polyester substrate causing bubbles in the adhesives, commonly used to adhere the substrate to a rigid support. This degradation and bubbling reduces the aesthetic and specular functional properties of the solar mirror.
Corrosion inhibitors and UV absorbers, incorporated into the adhesives or protective film coatings overlaying a polyester and silver mirror substrate, have been used to retain these functional properties. However, while corrosion inhibitors do reduce corrosion, they frequently impart an unacceptable color to the mirror, over time, and do not block the ultraviolet light. In contrast, when ultraviolet light absorbers are incorporated into a protective polymer overlay, the rate of polyester support degradation is lessened, but silver corrosion is aggravated. Thus, attempts have been made to isolate the corrosion inhibitor and ultraviolet absorber elements from the mirror's reactive components in order to eliminate these undesirable effects.
In Roche, U.S. Pat. No. 4,645,714, a corrosion resistant silver mirror is disclosed wherein a corrosion inhibitor, and an ultraviolet absorber are each incorporated into separate thin overlays of an acrylate inter-polymer paint. The specular reflective mirrors are formed by vapor-depositing silver over a polyester support film. Ultraviolet degradation of the polyester support, and consequent bubbling of the underlying adhesive, is reduced by incorporating UV absorbers in a second polymer coating that is applied over a first polymer coating, which incorporates a corrosion inhibitor. The first polymer coating is applied directly over the silver reflective surface. The polymeric substrate, a coextruded biaxially oriented polyester foil, comprises: (1) a polyethylene terephthalate lamina containing conventional slip agents to facilitate winding; and (2) a polyethylene terephthalate lamina containing no slip agent, which results in an optically-smooth exposed surface. The silver specular reflective layer overlies the smooth surface, of the coextruded film, and is bonded thereto. Layered over the silver is a first acrylate or methacrylate inter-polymer coating, having a 0.5 to 2.5% glycol dimercaptoacetate dispersant, which serves as a coupling agent, primer, and corrosion inhibitor. This coating weighs 1–4 g/m2. Overlying the first acrylate coating, is a second acrylate coating containing an ultraviolet absorber effective throughout the 300–400 nanometer range. The weight of the second coating is 4–8 g/m2. Inclusion of the corrosion inhibitor and the UV absorber into separate layers is designed to keep the UV absorber out-of-contact with the silver, and to avoid any corroding effect. On the opposite side of the coextruded polyester support is a uniform coating, weighing about 10–15 g/m2, of a tacky and pressure-sensitive adhesive (95:5 isooctyl acrylate:acrylamide copolymer). A conventional release liner, such as a silicone-coated polyester film, may be used to protect the adhesive prior to use. The disclosure of U.S. Pat. No. 4,645,714 is incorporated by reference as though fully set forth herein.
It is believed, however, by Hutchinson, U.S. Pat. No. 5,118,540, that the reflective films described in U.S. Pat. Nos. 4,307,150, and 4,645,714 are generally unsuitable for solar energy applications. Under outdoor conditions, the thin acrylate flood coat of these films tends to weather poorly and to quickly erode. These coatings thus offer an insufficient protective barrier to abrasion and moisture. Where the solar mirror comprises a substrate having a polyester support sheet and a layer of silver, as the outer acrylate flood coat, containing UV absorbers, erodes, ultraviolet light degrades the polyester support, and the mirror's aesthetic appearance and optical efficiency fail. In order to mitigate this problem, Hutchinson discloses the use of corrosion inhibitors and ultraviolet absorbers in an adhesive, which is used to bond an abrasion and moisture resistant fluorocarbon protective coating over a polyester and silver mirror substrate. The relevant embodiment, therein, describes a corrosion and ultraviolet light resistant flexible reflective film, where the respective inhibitors and absorbers are incorporated into separate coatings of an adhesive. A thin layer of silver is vapor-deposited on a flexible polyester support sheet, producing the specular silver surface. The adhesive is used to bond a fluorocarbon film, having an abrasion and weather resistant function, to the surface of the silver. The adhesive is applied in two separate layers. The first adhesive layer is adjacent to the silver deposit and contains a corrosion inhibitor. The second adhesive layer contains a UV absorber and overlays the first adhesive layer for use in bonding the fluorocarbon protective film to the silver surface. The use of adhesives to bond the fluorocarbon film to the silver surface is a required element of this construction, because fluorocarbon films do not bond to metal surfaces. However, this construction is not without its deficiencies when used, over time, as a solar mirror. Under ultraviolet light, the application of advanced adhesives (those incorporating UV absorbers and corrosion inhibitors) directly onto a silver substrate has resulted in degradation of the silver/adhesive interface. When silver is adhered directly to acrylic films tunnel and delamination failures have also occurred. Moreover, fluorocarbon protective films, without UV absorbers, often provide an insufficient weather resistant shield. For these reasons, it is believed that these silver mirrors when applied as solar reflectors remain lacking in long-term durability, which results in a loss of optical efficiency and aesthetic appearance.
In view of the foregoing considerations, a need therefore exists for a durable silver mirror, which is corrosion and weather resistant, effectively screens ultraviolet light, and retains its specular optical efficiency and aesthetic appearance when used as a component in solar reflectors.